Any way to make a 100K pot into a 50K?

[mike80]

I'm working on a buddy's Triple Rectifier, adding a bias pot. I need a 50K pot, but all I have at the moment is a 100K. He needs it for a gig tomorrow night, so I don't have time to order one from Mouser, and there isn't anyplace locally that sells a suitable pot.

I'm afraid the 100K will have way too much range and will be difficult to dial in precisely. Is there anyway to add a resistor in parallel or such to effectively drop it down to 50K?

[ohmslaw]

I'm working on a buddy's Triple Rectifier, adding a bias pot. I need a 50K pot, but all I have at the moment is a 100K. He needs it for a gig tomorrow night, so I don't have time to order one from Mouser, and there isn't anyplace locally that sells a suitable pot.

I'm afraid the 100K will have way too much range and will be difficult to dial in precisely. Is there anyway to add a resistor in parallel or such to effectively drop it down to 50K?

Put a 100K resistor in parallel with the circuit, maybe?

[vibroverbus]

stick a 100k across the pot. the taper won't be linear but it'll do the job no problem.

[edit] realized this might be underexplained... helpful to do sketch it on a piece of paper the math so you understand for future refs... if you strap the wiper for instance, with the wiper at one end you'll have 2 100k resistors in parallel, in the middle you'll have a '50k' and a 100k, at the other you've got '0k' and 100k.

you can strap the whole pot or strap the wiper to one end for different effects and depending on what the circuit configuration is. for simple 'variable resistor applications it doesn't matter too much, you just get slightly different taper with different methods.

but in places with 3 true connections where the pot resistance is used in the circuit at all times, if you strap the whole pot you'd be of course halving the resistance it sees. more problematically (or more cleverly if you want to do some cool stuff with tricky pot wiring) if you strap the wiper it suddenly makes the 'end-to-end' pot connection variable again and depending on which side you strap it'd go either up or down... this lets you set up a trick where one leg goes up in resistance while the wiper goes down for instance...

[zombiwoof]

Whew!...that's a bit confusing. To make a 100k pot into a 50k, you put a 100k resistor across the outside two legs(not the middle wiper connection) of the pot. You can put resistors from the outside legs to the wiper if you want to change the taper of the pot, but that's another story, and not what the poster is asking to do IMO.

Al

[mike80]

stick a 100k across the pot. the taper won't be linear but it'll do the job no problem.

[edit] realized this might be underexplained... helpful to do sketch it on a piece of paper the math so you understand for future refs... if you strap the wiper for instance, with the wiper at one end you'll have 2 100k resistors in parallel, in the middle you'll have a '50k' and a 100k, at the other you've got '0k' and 100k.

you can strap the whole pot or strap the wiper to one end for different effects and depending on what the circuit configuration is. for simple 'variable resistor applications it doesn't matter too much, you just get slightly different taper with different methods.

but in places with 3 true connections where the pot resistance is used in the circuit at all times, if you strap the whole pot you'd be of course halving the resistance it sees. more problematically (or more cleverly if you want to do some cool stuff with tricky pot wiring) if you strap the wiper it suddenly makes the 'end-to-end' pot connection variable again and depending on which side you strap it'd go either up or down... this lets you set up a trick where one leg goes up in resistance while the wiper goes down for instance...


:eek: That is a bit confusing.

Whew!...that's a bit confusing. To make a 100k pot into a 50k, you put a 100k resistor across the outside two legs(not the middle wiper connection) of the pot. You can put resistors from the outside legs to the wiper if you want to change the taper of the pot, but that's another story, and not what the poster is asking to do IMO.

Al

That's a little more simplified.

Thanks guys. I was trying to think of how it would be done all night at work last night. I figured adding a 100K resistor in parallel would do the trick, but I wasn't sure of where it would connect to.

[rockon1]

Ive read the usable range of a pot with a resistor across it is changed when you do this-or something to that affect? Bob

[vibroverbus]

Whew!...that's a bit confusing. To make a 100k pot into a 50k, you put a 100k resistor across the outside two

fair cop.... not my best explanatory moment. I should refrain from long posts when I'm thinking about 7 other things first thing in the morning.

but BTW, the variation you're referring to changes the taper as well. but as I said, in a case like this, nuance of taper wouldn't really matter.

[danieldroukas]

Yes, a 100K across the two outside terminals of the pot will make a 50k pot. However, to elaborate what vibroverbus mentioned in passing: "...the taper won't be linear." If you intend to use the pot as a volume control it will alter the way the volume swells in as you rotate the control. Normally a volume control is logarithmic, but doing what you're suggesting will make it more linear or even antilogarithmic (AKA reverse-audio), meaning that that the volume will increase a lot sooner than you may like.

If the pot is originally a linear taper pot, it will now certainly be antilogarithmic.

Here is a link to a picture I made a long time ago experimenting with the severity of antilogarithmic taper by the same method you're suggesting (a resistor in parallel with the pot). http://i61.photobucket.com/albums/h77/danieldroukas/misc/Pots.jpg

EDIT: I should mention the X-axes should be more appropriately labeled "percent through sweep: 0-100%"

[vibroverbus]

Here is a link to a picture I made a long time ago experimenting with the severity of antilogarithmic taper by the same method you're suggesting (a resistor in parallel with the pot). http://i61.photobucket.com/albums/h77/danieldroukas/misc/Pots.jpg

EDIT: I should mention the X-axes should be more appropriately labeled "percent through sweep: 0-100%"

hah-hah, I have exactly that same thing setup in a few spreadsheets to help remind me exactly what various pot mods do.